Mounted units are very useful. For me an ideal army would consist of nothing but ranged units and mounted units. Mounted swoop in and finish off wounded units, prevent melee from hitting your ranged units, pillage tiles, steal workers, capture cities and really do a number on your opponent's ranged units.
Also, once my army is on the way I will make a bunch of mounted units because they will join the battle quicker than anything else.
Composites, chariots & horses, Xbows & Knights, Artillery & Cavalry are really all you need to make for a successful assault. There is that weird time in there where muskets and canons are your best units but I tend to avoid war around then.
Also, the ability to spam chariots without horses is OP. Egypt and Attila are absurd.